Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce a directed EUV light beam include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser-produced-plasma (“LPP”), the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.
One particular LPP technique involves generating a stream of target material droplets and irradiating some or all of the droplets with laser light pulses, e.g. zero, one or more pre-pulse(s) followed by a main pulse. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a relatively short distance, e.g., 10-50 cm, from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., a focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. To efficiently reflect EUV light at near normal incidence, a mirror having a delicate and relatively expensive multi-layer coating is typically employed. Keeping the surface of the collector mirror clean and protecting the surface from plasma-generated debris has been one of the major challenges facing BUY light source developers.
In quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 10-200 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time.
For LPP light sources, it may be desirable to use one or more gases in the chamber for ion-stopping, debris mitigation, optic cleaning and/or thermal control. In some cases these gases may be flowing, for example, to move plasma generated debris, such as vapor and/or microparticles in a desired direction, move heat toward a chamber exit, etc. In some cases, these flows may occur during LPP plasma production. For example, see U.S. Ser. No. 11/786,145, filed on Apr. 10, 2007, now U.S. Pat. No. 7,671,349, issued on Mar. 2, 2010, hereby incorporated by reference herein. Other setups may call for the use of non-flowing, i.e., static or nearly static, gases. The presence of these gasses, whether static or flowing and/or the creation/existence of the LPP plasma may alter/effect each droplet as it travels to the irradiation region adversely affecting droplet positional stability.
In U.S. Ser. No. 12/214,736, filed on Jun. 19, 2008, entitled SYSTEMS AND METHODS FOR TARGET MATERIAL DELIVERY IN A LASER PRODUCED PLASMA EUV LIGHT SOURCE, 2006-0067-02, now U.S. Pat. No. 7,872,245, issued on Jan. 18, 2011, the use of a tube to envelop a portion of the droplet path as the droplets travel from a droplet release point to an irradiation region was described. As described, the tube was provided to shield and protect an optic such as a collector mirror from droplets/target material that strayed from the desired path between a droplet release point and the irradiation region, e.g. during droplet generator startup or shutdown. However, with the use of this continuous tube, unacceptable droplet positional instabilities were observed, specifically during plasma production.
With the above in mind, applicants disclose systems and methods for target material delivery protection in a laser produced plasma EUV light source, and corresponding methods of use.